The present invention relates in general to web forming processes and, more particularly, to improved cross machine direction control of such processes. While the present invention can be applied to a variety of systems, it will be described herein with reference to a web forming machine used for making sheets of paper for which it particularly applicable and initially being utilized.
Uniformity of a property of a web of sheet material can be specified as variations in two perpendicular directions: the machine direction (MD) which is in the direction of web movement during production and cross machine direction (CD) which is perpendicular to the MD or across the web during production. Different sets of actuators are used to control the variations in each direction. CD variations appear in measurements known as CD profiles and are typically controlled by an array of actuators located side-by-side across the web width. For example, in a paper making machine an array of slice screws on a headbox or an array of white-water dilution valves distributed across a headbox are usually used to control the weight profiles of webs of paper produced by the machine.
Control schemes are used to control the CD actuators in order to reduce the variations at different CD locations across the web. For such schemes to succeed, it is crucial to apply control adjustments to the correct actuators, i.e., actuators that control areas of the web in which CD variations are to be reduced. Hence, the spatial relationship between the CD location of an actuator and the area of the profile the actuator influences is key to the implementation of a high-performance CD controller. The cross direction spatial relationship, between CD actuators and a CD profile, is known to those skilled in the art as xe2x80x9cCD mappingxe2x80x9d. FIG. 1 shows an example of a CD mapping relationship 100 wherein bumps 102 made to actuators in an actuator array are reflected in the CD profile 106.
In many sheet-forming processes, the CD mapping relationship is not a linear function. For example, on a paper making machine, the CD mapping between the headbox slice screws and weight profile is particularly non-linear near the edges of the web due to the higher edge shrinkage. The nonlinear mapping relationship is a function of various machine conditions. The relationship cannot be easily represented with a fixed explicit function. Particularly in an ongoing web making operation where the CD mapping can change either gradually or abruptly, depending on the evolution of machine conditions.
Misalignment in the CD mapping can lead to deterioration in control performance. A typical symptom of mapping misalignment is the presence of sinusoidal variation patterns in both the CD profile and the actuator array. The appearance of the sinusoidal pattern is often referred to in the art as a xe2x80x9cpicket fencexe2x80x9d pattern. The picket fence cycles that appear in both the CD profile and actuator arrays occur in the same region of the sheet and are usually of comparable spatial frequencies. The pattern is caused by the control actions being applied to the misaligned actuators.
Although the mapping misalignment can be corrected by adjusting the control setup, in the past such adjustment has required manual intervention. Dependent on the frequency of CD mapping changes, the number of manual interventions may be significant. At a minimum, manual intervention requires determination of how wide the sheet is at the forming end (location of the process where the actuator array is situated) and at the finishing end (location of the process where the CD profiles are measured). While these determinations may be sufficient to satisfy processes with very minimal nonlinear shrinkage, for processes with extreme non-linear shrinkage, the scope of manual intervention may require perturbing the actuator array, at multiple locations, to determine the mapping relationship between the actuators and the CD profile. Such perturbations are typically performed with the CD control system turned off. Additionally, only a few actuators, spaced sufficiently far apart, are normally perturbed at a given time to ensure separation of the response locations in the CD profile. For a CD control system with a large actuator array, such perturbations or bumps may consume an extended period of production on the process.
It is also possible to control the smoothness of the setpoints of the actuator array, i.e., to restrict the setpoint differences between adjacent actuators in the actuator array, to reduce the amplitude of the cycles. Control of smoothness is also a mechanism for making the CD control system more robust for modeling uncertainty under different process conditions and the presence of uncontrollable variations in the CD profile.
Accordingly, there is a need in the art for an improved CD control for sheet making processes that can overcome changes in the mapping relationships between CD actuators and the corresponding CD profile of the web that they control. The control arrangement would correct the mappings without interruption of the CD control system and preferably would also control the smoothness of the setpoints of the actuator array instead of or in addition to corrections of the mappings.
This need is met by the invention of the present application wherein the CD profile of a web of material being produced is monitored and controlled to update CD control settings on-line so that changes in the operation of a machine manufacturing the web can be corrected before significant profile disturbances result. More particularly, detected variations in the profile that satisfy a search criteria, for example standard deviation between about 0.25% and about 0.75% of a web target or specification value, trigger searches for improved CD control settings. One aspect of the present invention recognizes CD actuator mapping misalignment, determines improved CD actuator control settings and applies the improved CD actuator control settings to fine tune a CD controller and thereby improve upon or correct the misalignment so that the CD controller will have improved and consistent long-term performance. Another aspect of the present invention recognizes abnormality in the smoothness of the setpoints of the CD actuators and controls the smoothness of the setpoints to again improve upon or correct such errors so that the CD controller will have improved and consistent long-term performance. The present invention encompasses the recognition and correction of either CD actuator mismatches or the CD actuator setpoint smoothness or both.
Features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.